Producing purified hydrocarbon gas from a gas stream comprising hydrocarbons and acidic contaminants

ABSTRACT

Process for producing purified hydrocarbon gas from a gas stream comprising hydrocarbons and acidic contaminants, which process comprises the steps: (a) cooling the gas stream to a temperature to form a mixture comprising solid and optionally liquid acidic contaminants and a vapour containing gaseous hydrocarbons; (b) separating the solid and optionally liquid acidic contaminants from the mixture in a vessel, yielding the purified hydrocarbon gas; (c) providing heat to at least a part of the solid and optionally liquid acidic contaminants to melt at least part of the solid acidic contaminants, yielding a heated contaminant-rich stream; (d) withdrawing the heated contaminant-rich stream from the vessel; wherein the process further comprises: (e) reheating at least a part of the heated contaminant-rich stream to form a reheated recycle stream; and (f) recycling at least a part of the reheated recycle stream to the vessel.

The present invention relates to a process for the removal of acidiccontaminants from a gas stream comprising hydrocarbons and acidiccontaminants. The invention especially relates to a process in whichcarbon dioxide and hydrogen sulphide are removed from natural gas thatcontains hydrocarbons and acidic contaminants.

Such a process is known from WO-A 2004/070297. This document discloses aprocess in which a natural gas stream comprising hydrocarbons and acidiccontaminants is first cooled in a first vessel to remove water from thenatural gas, and subsequently the natural gas is further cooled in asecond vessel to solidify acidic contaminants or dissolve suchcontaminants in a liquid, which contaminants are removed so that apurified natural gas is recovered. In the specification it isacknowledged that solid acidic contaminants may block the outlet of thesecond vessel. To prevent solid acidic contaminants from blocking suchoutlet a warm liquid comprising natural gas condensates may beintroduced into the lower part of the vessel so that at least part ofthe solid acidic contaminants melts.

In WO-A 2007/030888 a similar process for the removal of acidiccontaminants from natural gas is described. In this process the formedsolid acidic contaminants are heated to a temperature above the meltingpoint temperature of the contaminants by means of a heat exchanger inthe form of a bundle coil. The fluid that is passed through the bundlecoil can be the natural gas or any other process stream. Alternatively,a liquid process stream derived from another part of the process can bemixed with the solid acidic contaminants to melt these contaminants. Theaddition of a relatively warm stream to the solid acidic contaminantshas the advantage that it provides a more efficient and direct heattransfer than the indirect heat exchange via a bundle coil. However, byadding either a condensate stream or another process stream to the solidacidic contaminants, it may become necessary to separate these from thecontaminants since otherwise a significant loss of valuable hydrocarbonscould be incurred. Such separation needlessly complicates the process.The present invention has as objective to avoid such complications.

Accordingly, the invention provides a process for producing purifiedhydrocarbon gas from a gas stream comprising hydrocarbons and acidiccontaminants, which process comprises the steps:

-   -   (a) cooling the gas stream to a temperature to form a mixture        comprising solid and optionally liquid acidic contaminants and a        vapour containing gaseous hydrocarbons;    -   (b) separating the solid and optionally liquid acidic        contaminants from the mixture in a vessel, yielding the purified        hydrocarbon gas;    -   (c) providing heat to at least a part of the solid and        optionally liquid acidic contaminants to melt at least part of        the solid acidic contaminants, yielding a heated        contaminant-rich stream;    -   (d) withdrawing the heated contaminant-rich stream from the        vessel; wherein the process further comprises:    -   (e) reheating at least a part of the heated contaminant-rich        stream to form a reheated recycle stream; and    -   (f) recycling at least a part of the reheated recycle stream to        the vessel.

In the present process the reheated recycle stream is recycled to thevessel, to provide heat to the solid and optionally liquid contaminantsto melt at least part of the solid acidic contaminants. In this way thebenefits of direct heat exchange are obtained whilst no alien speciesare introduced into the mixture. Further, the process can be carried outalso when no condensates are available. Moreover, the present processavoids the need to provide for a complex heat exchanger in the lowerpart of he vessel.

The gas stream can be any stream of gas that comprises acidiccontaminants and hydrocarbons. In particular the process according tothe present invention can be applied to a natural gas stream, i.e., agas stream that contains significant amounts of methane and that hasbeen produced from a subsurface reservoir. It includes a methane naturalgas stream, an associated gas stream or a coal bed methane stream. Theamount of the hydrocarbon fraction in such a gas stream is suitably from10 to 85 mol % of the gas stream, preferably from 25 to 80 mol %.Especially, the hydrocarbon fraction of the natural gas stream comprisesat least 75 mol % of methane, preferably 90 mol %. The hydrocarbonfraction in the natural gas stream may suitably contain from 0 to 20 mol%, suitably from 0.1 to 10 mol %, of C₂ ⁻C₆ compounds. The gas streammay also comprise up to 20 mol %, suitably from 0.1 to 10 mol % ofnitrogen, based on the total gas stream.

Gas streams, such as natural gas streams, may become available at atemperature of from −5 to 150° C. and a pressure of from 10 to 700 bar,suitably from 20 to 200 bar. In the process of the present invention thegas stream comprises suitably hydrogen sulphide and/or carbon dioxide asacidic contaminants. It is observed that also minor amounts of othercontaminants may be present, e.g. carbon oxysulphide, mercaptans, alkylsulphides and aromatic sulphur-containing compounds. The major part ofthese components will also be removed in the process of the presentinvention.

The amount of hydrogen sulphide in the gas stream containing methane issuitably in the range of from 5 to 40 volume % of the gas stream,preferably from 20 to 35 volume % and/or the amount of carbon dioxide isin the range of from 10 to 90 vol %, preferably from 20 to 75 vol %,based on the total gas stream. It is observed that the present processis especially suitable for gas streams comprising large amounts ofcontaminants, e.g. 10 vol % or more, suitably between 15 and 90 vol %.

Gas stream containing the large amounts of contaminants as describedabove cannot be processed using conventional techniques as amineextraction techniques as they will become extremely expensive,especially due to the large amounts of heat needed for the regenerationof loaded amine solvent.

The gas stream, and in particular natural gas streams produced from asubsurface formation, may typically contain water. In order to preventthe formation of gas hydrates in the present process, at least part ofthe water is suitably removed. Therefore, the natural gas stream that isused in the present process has preferably been dehydrated. This can bedone by conventional processes. A suitable process is the one describedin WO-A 2004/070297. Other processes for forming methane hydrates or fordrying natural gas are also possible. Other dehydration processesinclude treatment with molecular sieves or drying processes with glycolor methanol. Suitably, water is removed until the amount of water in thenatural gas stream comprises at most 50 ppmw, preferably at most 20ppmw, more preferably at most 1 ppmw of water, based on the totalnatural gas stream.

As indicated above, acidic contaminants that are usually present innatural gas streams include hydrogen sulphide and carbon dioxide. It isalso possible that the natural gas stream contains other components,including ethane, propane and hydrocarbons with four or more carbonatoms, even after an optional earlier recovery of condensates. It willbe appreciated that when a portion of acidic contaminants, e.g., carbondioxide, solidifies in the cooling stage, other components, e.g.,hydrogen sulphide and hydrocarbons other than methane, may liquefy. Theliquid components are suitably removed together with the solid acidiccontaminants from the vapour.

In a first step of the present process the gas stream is cooled. Thecooling may be effected by any known method, such as indirect heatexchange and expansion. Alternatively, a direct heat exchange, e.g., byspraying with a cold liquid, as shown in WO-A 2004/070297, is alsopossible. The skilled person will appreciate that expansion causes alowering of temperature, so that cooling may be achieved by expansionand adapting pressure. Preferably the expansion is done by isenthalpicexpansion, preferably isenthalpic expansion over an orifice or a valve,especially a Joule-Thomson valve or a series of Joule-Thomson valves. Inanother preferred embodiment the expansion is done by nearly isentropicexpansion, especially by means of an expander, preferably a turboexpander, or a laval nozzle. The cooling may be conducted in severalsteps. It is preferred that the gas stream is subjected to heat exchangewith one or more other cold process streams or external streams. Coldexternal streams may be suitably streams from an LNG (liquefied naturalgas) plant, such a cold LNG stream or a refrigerant stream, or from anair separation unit. One suitable stream comprises the purifiedhydrocarbon gas. At such cooling hydrocarbons may condense and suchliquid condensate may be recovered before the gas stream is cooledfurther to the temperature at which acidic contaminants solidify.Preferably, the cooling stage of the natural gas stream comprises one ormore expansion steps. For this purpose conventional equipment may beused. Conventional equipment includes turbo-expanders, so-calledJoule-Thomson valves and venturi tubes. It is preferred to at leastpartly cool the gas stream over a turbo-expander, releasing energy. Oneadvantageous effect of using the turbo-expander is that the energy thatis released in the turbo-expander can suitably be used for compressingat least part of the purified hydrocarbon gas. Since the stream of thepurified hydrocarbon gas is smaller than the gas stream now that acidiccontaminants have been removed, the energy is suitably such that thepurified hydrocarbon gas may be compressed to an elevated pressure thatmakes it suitable for transport in a pipeline.

The cooling steps eventually lead to the desired temperature at whichacidic contaminants solidify. However, since the natural gas stream alsomay comprise hydrocarbons other than methane it is preferred to cool thenatural gas stream, suitably by expansion, to a temperature below thedew point of propane. In this way the vaporous natural gas stream willdevelop liquid hydrocarbons, including propane, which can subsequentlybe recovered easily from the vapour.

It is preferred to achieve the cooling in several steps, e.g., byindirect heat exchange and/or expansion. It is also possible to solidifyby spraying with a cold liquid, as shown in WO-A 2004/070297. Suitably,solid acidic contaminants are obtained in a final expansion step. Thefinal expansion step is preferably achieved over a Joule-Thomson valve.Therefore, preferably, in a first step, which may be achieved by variousintermediate steps and various methods, the gas stream is cooled to atemperature ranging from 1 to 40° C. above the freeze out temperature ofthe first acidic contaminant to freeze out, the freeze out temperaturebeing the temperature at which solid contaminants are formed.Preferably, the cooling is effected till from 2 to 10° C. above thefreeze out temperature. In a final step the gas stream is preferablycooled to the temperature at which a mixture of solid and/or liquidacidic contaminants and a vapour comprising gaseous hydrocarbons areformed by expansion over a valve. Preferably, the gas stream is partlyor completely liquid before being expanded over the valve, and solidcontaminants are formed upon expansion. This ensures better separationperformance in the vessel. Suitably, the gas stream is expanded from apressure ranging from 40 to 200 bar to a pressure of 10 to 40 bar.Expansion over this pressure range suitably causes that solid acidiccontaminants are formed. It will be appreciated by the person skilled inthe art that at the formation of solid acidic contaminants also liquidacidic contaminants may be formed and/or hydrocarbons may condense.These liquid components are suitably separated together with the solidacidic contaminants.

The solidification of acidic contaminants may take place very rapidly,especially upon expansion over a valve, thereby forming the mixturecomprising solid and optionally liquid acidic contaminants and a vapourcomprising gaseous hydrocarbons. To facilitate the separation themixture is passed into a vessel wherein the separation between solidacidic contaminants and vapour takes place. By gravity the solid acidiccontaminants, and any liquid that is formed, drop to the bottom of thevessel. After such separation the solid acidic contaminants can beremoved from the process.

After separation of solid and optionally liquid acidic contaminants, thepurified hydrocarbon gas that is being recovered after the separationstep can be used as product. The recovered purified hydrocarbon gas mayalso be subjected to further treatment and/or purification. Forinstance, the purified hydrocarbon gas may be subjected tofractionation. In the event that the purified hydrocarbon gas is naturalgas intended for pipeline transportation or for producing liquefiednatural gas (LNG),in order to reach pipeline specifications or LNGspecifications the purified natural gas may further purified. Furtherpurification can for example be done in an additional cryogenicdistillation column, suitably with a bottom temperature between −30 and10° C., preferably between −10 and 5° C. A reboiler may be present tosupply heat to the column. Suitably the top temperature column isbetween −110 and −80° C., preferably between −100 and −90° C. In the topof the cryogenic distillation column a condenser may be present, toprovide reflux and a liquefied (LNG) product.

As an alternative, further purification may be accomplished byabsorption with a suitable absorption liquid. Suitable absorbing liquidsmay comprise chemical solvents or physical solvents or mixtures thereof.

A preferred absorbing liquid comprises a chemical solvent and/or aphysical solvent, suitably as an aqueous solution.

Suitable chemical solvents are primary, secondary and/or tertiaryamines, including sterically hindered amines.

A preferred chemical solvent comprises a secondary or tertiary amine,preferably an amine compound derived from ethanolamine, more especiallyDIPA, DEA, MMEA (monomethyl-ethanolamine), MDEA (methyldiethanolamine)TEA (triethanolamine), or DEMEA (diethyl-monoethanolamine), preferablyDIPA or MDEA. It is believed that these chemical solvents react withacidic compounds such as CO₂ and H₂S.

Suitable physical solvents include tetramethylene sulphone (sulpholane)and derivatives, amides of aliphatic carboxylic acids, N-alkylpyrrolidone, in particular N-methyl pyrrolidine, N-alkyl piperidones, inparticular N-methyl piperidone, methanol, ethanol, ethylene glycol,polyethylene glycols, mono- or di(C₁-C₄)alkyl ethers of ethylene glycolor polyethylene glycols, suitably having a molecular weight from 50 to800, and mixtures thereof. The preferred physical solvent is sulfolane.It is believed that CO₂ and/or H₂S are taken up in the physical solventand thereby removed.

Other treatments may include a further compression, when the purifiedhydrocarbon gas is wanted at a higher pressure. If the amounts of acidiccontaminants in the purified hydrocarbon gas are undesirably high, thepurified hydrocarbon gas may be subjected to one or more repetitions ofthe present process.

In the event that the hydrocarbon gas is natural gas, the purifiednatural gas can be processed further in known manners, for example bycatalytic or non-catalytic combustion to produce synthesis gas, togenerate electricity, heat or power, or for the production of liquefiednatural gas (LNG), or for residential use. It is an advantage of thepresent process enables purification of natural gas comprisingsubstantial amounts of acidic contaminants, resulting in purifiednatural gas comprising low levels of contaminants, especially of sulphurcontaminants. The production of LNG from such natural gas, which wouldbe very difficult if not impossible by conventional processes, is madepossible. Thus, the invention also provides LNG obtained from liquefyingpurified natural gas obtained by the process. The LNG thus-obtainedtypically has very low concentrations of contaminants other than naturalgas.

Since it is easier to transport liquids than to transport solids, it ispreferred to melt at least partly the solid acidic contaminants.Therefore, it has been proposed to heat at least a part of the solidacidic contaminants to cause melting, thereby yielding the heatedcontaminant-rich stream that is withdrawn from the vessel bottom,suitably by pumping.

According to the present process at least a part of the heatedcontaminant-rich stream is reheated to yield a reheated recycle stream.The recycle of part of the reheated recycle stream is meant to melt atleast part of the solid acidic contaminants in the vessel so thatblocking is prevented and removal of the acidic contaminants isfacilitated. Preferably, the heat that is being provided by the recycledreheated recycle stream is such that it causes the melting of all solidacidic contaminants. The skilled person may achieve this by selectingthe desired temperature of the reheated recycle stream and/or the amountof reheated recycle stream. Therefore, the part of the contaminant-richstream that is reheated to form the reheated recycle stream ispreferably heated to form a liquid stream, more preferably without anysolid acidic contaminant. Suitably the heating up is done to atemperature well above the melting point of the solid acidiccontaminants, such as at least 5° C. above the highest melting point.The heat of the relatively warm liquid will melt at least part of thesolid acidic contaminants in the vessel. It is even more preferred thatthe part of the heated contaminant-rich stream that is reheated to formthe reheated recycle stream is heated to such a temperature that thestream becomes at least partly vaporous. Not only will more energy berecycled to the vessel so that the melting of solid acidic contaminantsis conducted more smoothly, but also any light hydrocarbon that may beentrained in the heated contaminant-rich stream will be freed up and canbe included in the purified hydrocarbon gas that is withdrawn from thevessel. In this way the recovery of purified hydrocarbon gas isenhanced.

In order to improve the heat transfer between the warm fluid, i.e.either liquid or vaporous, and the cold solid and optionally liquidcontaminants, the vessel is preferably provided with internals. Theseinternals will increase the contacting surface between cold solids andwarm fluid as well as provide residence time to the components in thevessel so that acidic contaminants may condense and/or solidify andliquid hydrocarbons may be evaporated. The skilled person may select theinternals from a variety of possibilities. Very suitable are sieveplates, perforated plates or bubble trays. Their construction isrelatively easy in the cryogenic environment of the vessel, whereas thecontacting performance is very good.

An even more preferred embodiment comprises a vessel that has beenprovided with at least one deflecting means that has been arranged inthe interior of the vessel. Downwards-falling solid and liquid acidiccontaminants are distributed more homogeneously over the cross-sectionof the vessel, thereby improving the separation between solid and acidiccontaminants on the one hand and the gaseous hydrocarbons on the other.The shape of the deflecting means can be selected from a variety ofshapes; the deflecting means may, e.g., be square, circular or of aring-shape. Preferably the deflecting means has downwards-directedslopes to avoid build up of solid material on the deflecting means. Avery suitable shape is a cone or a combination of a cone and an invertedcone. Whereas the cone ensures a smooth distribution of solid and liquidmaterial, the inverted cone provides for a suitable passage forupwards-flowing gases. The deflecting means suitably covers from 5 to75% of the cross-section of the vessel.

It is evident that the reheating of part of the heated contaminant-richstream requires energy. In a preferred embodiment a part of the heatedcontaminant-rich stream is separated and this part of the heatedcontaminant-rich stream is reheated to form the reheated recycle stream.In this way only a portion of the heated contaminant-rich streamrequires to be heated up. The size of the part of the heatedcontaminant-rich stream that is separated can be selected by the skilledperson depending on conditions such as the temperature of the reheatedrecycle stream and the amount and nature of the solid acidiccontaminants. Suitably, the part of the heated contaminant-rich streamthat is separated is selected such that the reflux ratio ranges from 0.5to 10. This embodiment is especially advantageous when the part of theheated contaminant-rich stream is heated up to form a vaporous recyclestream.

In another embodiment of the present invention substantially the entireheated contaminant-rich stream that is withdrawn from the vessel isreheated to form the reheated recycle stream, and a part of the thusobtained reheated recycle stream is recycled to the mixture. Thisembodiment is especially useful when the heated contaminant-rich streamis heated up to a liquid. A part of the reheated recycle stream isrecycled, whereas the other part is withdrawn, optionally after recoveryof entrained hydrocarbons. The size of the part that is being recycledcan be determined by the skilled person, based on the conditions, alsoindicated above. Suitably, the part of the reheated recycle stream thatis recycled to the mixture is selected such that the reflux ratio rangesfrom 0.5 to 10.

The way in which the contaminant-rich stream is reheated can be done inany feasible way. External, e.g., electrical, heaters are possible.However, preferably, the reheating of at least part of thecontaminant-rich stream is conducted via heat exchange. Any processstream with a sufficiently higher temperature can be used for this. Thisincludes any condensate stream or hydrate-stream. Preferably, the heatexchange is conducted with at least part of the gas stream.Alternatively, a warm fluid may be added to the contaminant-rich stream.Suitable warm fluids include a natural gas condensate.

In the event that the contaminant-rich stream mainly comprises carbondioxide and is therefore a CO₂-rich stream, preferably CO₂-rich streamis further pressurised and injected into a subterranean formation,preferably for use in enhanced oil recovery or for storage into anaquifer reservoir or for storage into an empty oil reservoir. It is anadvantage that a liquid CO₂-rich stream is obtained, as this liquidstream requires less compression equipment to be injected into asubterranean formation.

The process will be explained in more detail by means of the followingfigures.

FIG. 1 shows a schematic embodiment of a vessel wherein a recycle streamis being applied.

FIG. 2 shows another embodiment of such a vessel.

FIG. 3 shows a schematic flow scheme of a natural gas purification unitusing the process of the present invention.

FIG. 1 shows a vessel wherein a dehydrated natural gas stream is cooledby expansion in a Joule-Thomson valve 2. Alternatively, instead of aJoule-Thomson valve a venturi tube or a turbo-expander may be used. Thethus cooled mixture of solids and vapour is passed through a line 3 to avessel 4. Line 3 that connects the valve 2 with the vessel 4 is short sothat the solids will not block the entry of the mixture to the vessel 4.It is also possible to do away with the line 3 altogether and connectthe Joule Thomson valve directly to the wall of vessel 4. The cooledmixture is separated in vessel 4 to a purified hydrocarbon gas thatexits the vessel 4 via an outlet 5. Solid acidic contaminants, and anyliquid components, fall down via a deflecting means 15 to the lower partof the vessel 4 forming a slurry containing solid acidic contaminants 6.Deflecting means 15 has the shape of a cone. Via a line 9 a warm recyclestream is recycled into the vessel 4. Optionally an additionalelectrical immersion heater or bundle coil 14 through which a warm fluidis passed, may be provided to provide additional energy. Due to therecycle of warm fluids solid acidic contaminants melt and the remainingliquid, i.e., the heated contaminant-rich stream, is withdrawn from thevessel 4 via a line 7, optionally by pumping. The entire heatedcontaminant-rich stream in line 7 is subjected to heat exchange in heatexchanger 10, through which a warm fluid, e.g. the natural gas stream,is being passed to reheat the contaminant-rich stream. A part of thethus reheated stream is withdrawn from the process via a line 11.Another part is recycled to the vessel via line 9. The part that isbeing recycled can be fed into the vessel 4 in any known way. Hence, itis possible to use a single nozzle, or a plurality of nozzles, one ormore spargers, or nozzles arranged in a supply line that extends intothe vessel, which supply line may be ring-shaped. The lines 9 and 11have been provided with valves 12 and 13 to control the flow of recyclestream to the vessel 4 and the liquid level inside vessel 4. The line-upof FIG. 1 is especially suitable for the situations in which thereheated recycle stream is maintained in the liquid phase.

In FIG. 2 a situation is shown that is especially suitable forsituations wherein recycle streams are reheated to become vaporous.Similar to the embodiment of FIG. 1, a natural gas stream is passed viaa line 21 and a Joule-Thomson valve 22 and another short line 23 to avessel 24. Alternatively, a venturi tube may be used instead of aJoule-Thomson valve. Purified hydrocarbon gas is removed via an outlet25. Solid acidic contaminants and optionally also some liquids fall downvia a deflecting means 36 and are collected as layer 26 in the bottompart of vessel 24. The deflecting means 36 in this embodiment has beenexecuted as a combination of a cone and an inverted cone. The layer 26comprising solid acidic contaminants can be heated to melt at least partof the solid acidic contaminants by the recycle of a reheated recyclestream via a nozzle 32. An additional heater 35 may optionally beprovided. Due to the energy input of the recycle and/or heating solidacidic contaminants melt, and the remaining heated contaminants-richstream is withdrawn from the vessel via line 27, optionally via pumping.The heated contaminant-rich stream is split into stream 29, which isdiscarded or is sent to another part of the process, and stream 28,which is subjected to heat exchange in a heat exchanger 30. The heatexchanger is similar to heat exchanger 10 in FIG. 1. A reheated recyclestream exits heat exchanger 30 via line 31. Line 31 debouches into thenozzle 32 through which the reheated recycle stream enters the vessel24. Both lines 31 and 29 have been provided with valves 33 and 34,respectively, to control the flow of recycle stream to the vessel 24 andthe liquid level inside vessel 24.

FIG. 3 shows a more extensive flow scheme of a unit wherein the presentprocess can be carried out.

A natural gas stream is introduced via a line 101 into a dehydratingunit 118. In the dehydration unit 118 water is being removed from thenatural gas stream, e.g., by means of molecular sieves. The water iseventually removed via a line 102. The dehydrated natural gas is passedvia a line 103 to a turbo-expander 119 where it is cooled, andsubsequently forwarded via a line 104. The natural gas in line 104 iscooled further via a heat exchanger 122. Subsequently, the natural gasstream is passed via a line 105 for further heat exchange. To recover asmuch energy as possible the natural gas stream may be passed to a heatexchanger 124 wherein it exchanges heat with purified hydrocarbon gasand heated contaminant-rich stream. If desired further heat exchange mayoptionally be established in heat exchanger 125. Via a line 106 and laline 107 the further cooled natural gas stream is passed to aJoule-Thomson valve 126 where it is cooled to a temperature at whichacidic contaminants solidify so that a mixture of solid acidiccontaminants and vaporous hydrocarbons enter vessel 120 via a line 108.Alternatively, a venturi tube may be used instead of a Joule-Thomsonvalve. There separation takes place so that purified hydrocarbon gas iswithdrawn at the top via a line 109. The figure shows that short line108 connects the Joule Thomson valve 130 with the vessel 120. This lineis typically short so that blocking of the line by solids is prevented.It is also possible to do away with the line altogether and connect theJoule Thomson valve directly to the wall of vessel 120.

The solid and optionally liquid acidic contaminants and optionallyliquid hydrocarbons are falling down, preferably along a deflectingmeans (not shown), towards the bottom of vessel 120 where they arecollected and heated by means of a warm reheated recycle stream enteringthe vessel 120 via a line 117, thereby melting solid acidiccontaminants. The thus obtained heated contaminant-rich stream iswithdrawn from the vessel via a line 112 which is pumped further usingpump 121. The heated contaminant-rich stream is divided into a part thatis withdrawn via a line 113 and a part that is forwarded to the heatexchanger 122 via a line 115. In heat exchanger 122 the part of theheated contaminant-rich stream is reheated by means of heat exchangewith the natural gas stream provided via line 104, to form a reheatedrecycle stream. The reheated recycle stream is forwarded via line 116 toa valve 123 which controls the flow of the reheated recycle stream. Viathe line 117, which may be provided with a nozzle (not shown) thereheated recycle stream is introduced into vessel 120.

The line 113 with the heated contaminant-rich stream leads the moltencontaminants to the heat exchanger 124, and subsequently, thecontaminants are withdrawn via line 114. In heat exchanger 124 themolten contaminants in line 113 and cold purified hydrocarbon gas inline 109 are subjected to heat exchange with the natural gas stream inline 105. The streams are shown in co-current fashion. It is evident tothe skilled person that the streams may also be arranged in acounter-current way, e.g., the relatively warm natural gas stream iscounter-current with the two other streams. It will be appreciated thatit is also feasible to use only one of the other streams or use a streamfrom another process, such as a stream from an LNG plant and/or an airseparation plant.

From the heat exchanger 124 the purified hydrocarbon gas is passed via aline 110 to a compressor 127. The compression energy for compressor 127is suitably provided by the expander 119. The compressed gas may berecovered as product in line 111 or used for further treatment.

1. Process for producing purified hydrocarbon gas from a gas streamcomprising hydrocarbons and acidic contaminants, which process comprisesthe steps: (a) cooling the gas stream to a temperature to form a mixturecomprising solid acidic contaminants and a vapour containing gaseoushydrocarbons; (b) separating the solid acidic contaminants from themixture in a vessel, yielding the purified hydrocarbon gas; (c)providing heat to at least a part of the solid acidic contaminants tomelt at least part of the solid acidic contaminants, yielding a heatedcontaminant-rich stream; (d) withdrawing the heated contaminant-richstream from the vessel; (e) reheating at least a part of the heatedcontaminant-rich stream to form a reheated recycle stream; and (f)recycling at least a part of the reheated recycle stream to the vessel.2. Process as claimed in claim 1 in which the part of the heatedcontaminant-rich stream that is reheated to form the reheated recyclestream is heated to form a liquid stream.
 3. Process as claimed in claim1, in which the part of the heated contaminant-rich stream that isreheated to form the reheated recycle stream is heated to such atemperature that the stream becomes at least partly vaporous.
 4. Processas claimed in claim 1, in which a part of the heated contaminant-richstream is separated and this part of the heated contaminant-rich streamis reheated to form the reheated recycle stream.
 5. Process as claimedin claim 4, in which the part of the heated contaminant-rich stream thatis separated is selected such that the reflux ratio ranges from 0.5 to10.
 6. Process as claimed in claim 1, in which substantially the entireheated contaminant-rich stream that is withdrawn from the vessel isreheated to form the reheated recycle stream, and a part of the thusobtained reheated recycle stream is recycled to the mixture.
 7. Processas claimed in claim 6, in which the part of the reheated recycle streamthat is recycled to the mixture is selected such that the reflux ratioranges from 0.5 to
 10. 8. Process as claimed in claim 1, in which thereheating of at least part of the heated contaminant-rich stream isconducted via heat exchange, in which the heat exchange is preferablyconducted with at least part of the gas stream.
 9. Process as claimed inclaim 1, in which the reheating of at least part of the heatedcontaminant-rich stream is conducted by adding thereto a warm fluid,preferably natural gas condensate.
 10. Process as claimed in claim 1, inwhich the cooling of the gas stream utilises one or more heat exchangesteps.
 11. Process as claimed in claim 1, in which the cooling of thegas stream utilises one or more expansion steps.
 12. Process as claimedin claim 1, in which the gas stream has been dehydrated, preferably to awater content of less than 50 ppmw, based on the total gas stream. 13.Process as claimed in claim 12, in which gas stream is expanded from apressure ranging from 40 bar to 200 bar, to a pressure of 10 bar to 40bar.
 14. Process as claimed in claim 1, wherein the purified gas ispurified natural gas and the process further comprises the steps ofcooling the purified natural gas to obtained liquefied natural gas. 15.(canceled)